1. Field of the Invention
This invention relates to a misfire-detecting system for internal combustion engines, and more particularly to a misfire-detecting system of this kind, which is adapted to detect a misfire attributable to the fuel supply system.
2. Prior Art
An internal combustion engine has spark plugs provided for cylinders for igniting a mixture of fuel and air drawn into the respective cylinders. In general, high voltage (sparking voltage) generated by the ignition coil of the engine is sequentially distributed to the spark plugs of the cylinders of the engine via a distributor, to ignite the air-fuel mixture. If normal ignition does not take place at one or more of the spark plugs, i.e. a misfire occurs, it will result in various inconveniences such as degraded driveability and increased fuel consumption. Furthermore, it can also result in so-called after-burning of unburnt fuel gas in the exhaust system of the engine, causing an increase in the temperature of a catalyst of an exhaust gas-purifying device arranged in the exhaust system. Therefore, it is essential to prevent occurrence of a misfire. Misfires are largely classified into ones attributable to the fuel supply system and ones attributable to the ignition system. Misfires attributable to the fuel supply system are caused by the supply of a lean mixture or a rich mixture to the engine, while misfires attributable to the ignition system are caused by failure to spark (so-called mis-sparking), i.e. normal spark discharge does not take place at the spark plug, due to smoking or wetting of the spark plug with fuel, particularly adhesion of carbon in the fuel to the spark plug, which causes current leakage between the electrodes of the spark plug, or abnormality in the ignition system.
The present assignee has already proposed a misfire-detecting system for detecting misfires attributable to the fuel supply system, which comprises sparking voltage detecting means, and misfire-determining means which determines occurrence of a misfire based on results of comparison between the detected value of the sparking voltage and a predetermined reference value (Japanese Provisional Patent Publication (Kokai) No. 4-279768), and further a misfire-determining system of this kind which comprises sparking voltage-detecting means, and misfire-determining means which determines that a misfire has occurred when a time period (hereinafter referred to as "high voltage time period") over which the detected value of the sparking voltage exceeds a predetermined voltage value or a value proportional to an area (hereinafter referred to as "high voltage area") of a portion of the detected sparking voltage exceeding the predetermined voltage value exceeds a reference value (U.S. Ser. No. 07/846,238 filed Mar. 5, 1992 based on Japanese Patent Application No. 3-67940).
However, according to the above proposed systems, if there exists a portion of the secondary side of the igniting circuit which is low in insulation, the sparking voltage does not rise to a level required for causing dielectric breakdown between the electrodes of the spark plug. As a result, there occurs almost no difference in the sparking voltage, the high voltage time period, or the high voltage area, between when a misfire occurs and when normal firing occurs, which prevents an accurate determination of occurrence of a misfire.
Further, the present assignee has proposed a misfire-detecting system in which a diode is incorporated in the secondary side of the igniting circuit, which serves to prolong the high voltage time period at a misfire to thereby achieve more accurate misfire determination, e.g. by U.S. Ser. No. 07/846,309 filed Mar. 5, 1992. According to this proposed system, however, if the diode is short-circuited, the system can only exhibit the same degree of misfire-detecting accuracy as a system not employing such an diode.
Further, if the spark plug smolders, it is also impossible to achieve an accurate misfire determination based on the high voltage time period, etc.